The present invention relates to a touch-operated see-through input unit of electrode-contact type or, more particularly, to a touch-operated see-through coordinate input unit of practically desirable performance which can be operated easily and reliably without giving little fatigue to the operator even after a long time of continued operation.
Various types of coordinate input units are known and used for inputting data to electronic instruments including the electrode-contact type, pressure type and surface acoustic wave type ones. When such a input unit is used as mounted on or in front of the display screen of a CRT display unit for computer terminals, it is sometimes desirable that the input unit is see-through in order not to disturb the operator's sight on the display screen. From the standpoint of satisfying such a requirement, conventional coordinate input units are not quite satisfactory in several respects. For example, the electrode-contact type unit is a membrane type key board switching unit formed of transparent plastic films provided with an extremely thin and light-transmitting electroconductive surface film of a metal, e.g. silver and lead, or an electroconductive metal oxide, e.g. indium oxide, by vapor-phase deposition or sputtering on certain areas so that, although such a unit is simple in the structure and widely used industrially, several disadvantages are unavoidable that the transparency is not high enough, the contact resistance is sometimes high and the electroconductive surface film is mechanically fragile leading to a limited yield of acceptable products in manufacture and low durability or serviceable life.
Further, touch-operated see-through coordinate input units are expected to find wide applications in many fields in which intensive investigations are under way for development using such a unit, for example in the systems of plant control of which high reliability is essential, instruments for office automation of which good operability is required to facilitate long-time continued operation, instruments of which size reduction is required such as portable type terminal instruments, instruments operated by many and unspecified persons such as terminal instruments of a videotex system and the like.
A type of touch-operated see-through coordinate input units is disclosed in Japanese Utility Model Publication 60-9869 according to which a flexible insulating sheet having transparency and a transparent insulating substrate plate are held in parallel and a first array of metal wires each in parallel to the others at a uniform pitch and a second array of metal wires each in parallel to the others at a uniform pitch are disposed between the insulating sheet and the insulating substrate plate in such a lattice-wise manner that the wires in the first array and the wires in the second array are perpendicular to each other while an insulating string having a diameter larger than the metal wires in the first and second arrays is disposed in every space between the adjacent metal wires in the first or second array to serve as a spacer for keeping the metal wires in the first and the second arrays apart from each other to ensure electric insulation therebetween when the input unit is not pushed by finger touch. The coordinate input unit of this type, however, is disadvantageous because of the low reliability caused by the intrusion of atmospheric dusts between the sheet and substrate plate and improvement in the reliability of contacting cannot be expected by the impression of a large electric current.